Introduction Indoor environmental quality (IEQ) is particularly important in classrooms, as students spend a significant portion of their time there. This study analyzes the variability of IEQ in a naturally ventilated classroom during the heating season. It assesses environmental quality using a newly developed 10-point IEQI index that integrates indoor air pollutant concentrations and thermal conditions. Methods Low-cost sensors were applied to continuously monitor typical indoor air pollutant concentrations and thermal condition parameters. The obtained results were used to calculate the Indoor Air Quality Index (IAQI) and the Thermal Condition Index (TCI), which served as the basis for determining the overall IEQ Index (IEQI). Variations in the IEQI were analyzed during periods of student occupancy, during the night, and throughout the entire monitoring period. Results When students were present in the classroom, pollutant concentrations, particularly CO 2 , increased significantly, with maximum values often exceeding 3300 ppm. Air temperature rose above 24°C. The calculated IAQI, TCI, and IEQI values were sensitive to occupancy fluctuations and varied throughout the day. IEQI values sometimes exceeded level 5, indicating relatively poor environmental quality at these times. Discussion The findings are consistent with previous studies that indicate that student presence negatively affects indoor environmental conditions in classrooms. The proposed IEQI, reflecting the dynamic interplay of occupancy, air pollution, and thermal conditions, allows for a relatively simple and ongoing assessment of environmental quality in classrooms. To fully evaluate its reliability, interpretability, and practical value, subjective validation of the IEQI is recommended. Conclusion Student presence significantly contributes to the deterioration of IEQ in the classroom. The proposed IEQI is a promising tool for monitoring and improving indoor environmental conditions not only in the studied classroom but also in similar spaces. By accounting for all relevant environmental parameters and conducting appropriate validation, this index can support the optimization of HVAC systems and the implementation of intelligent indoor environmental control strategies, thereby ensuring healthier, more comfortable conditions for students in classrooms.
Polednik et al. (Tue,) studied this question.